The current requirements of the mechanical industries are growing in terms of materials that withstand severe conditions of use. Numerous tools, such as the tools for cutting, shaping, milling, and stamping, molds, roller bearings, etc., comprise metal parts that are subjected during their use to significant wear, to corrosion, or to high-temperature oxidation. The problems of wear, for example, bring about mechanical shutdowns and significant costs for repair and replacement of parts. To extend the service life of these metal parts—made of steel or alloys—it is possible to coat them with a fine layer, typically of several microns, of a non-oxide ceramic compound such as carbide, nitride, or carbonitride of a metal element, which improves their mechanical properties as well as their resistance to wear and to corrosion. Chromium is the most used metal element, but other transition metals that have similar properties are also used. The nanostructured coatings generate a high interest today because they are able to protect tools that operate under extreme conditions of wear and aggressive environments.
To be effective, the deposition is to be uniform over the entire surface, despite the large size of the parts; it is to be free of oxide and produced by techniques that do not interfere with functional properties of the treated tools. In particular, the temperature conditions during the deposition should lead neither to a deformation of parts, nor to a structural transformation. In addition, a process is sought whose conditions of use can be easily industrialized and economically inexpensive. From this standpoint, the possibility of laying down deposits for atmospheric-pressure protection is of primary importance because it makes it possible to consider online treatment of large parts in a stream. Finally, the process that is as least polluting as possible is sought. The processes for wet deposition (treatment by immersion in a bath or electrodeposition) are therefore prohibited because of the effluent wastes loaded with toxic compounds that they generate. They will also soon be prohibited in Europe.
Obtaining non-oxide ceramic-type coatings by dry deposition techniques is well known. Among the latter, chemical deposition techniques (CVD, for Chemical Vapor Deposition) and physical chemical deposition techniques (PVD, for Physical Vapor Deposition) have been mastered and are already used in production.
For example, the chemical vapor-phase deposition of nitrides, carbides or carbonitrides of metal elements from a cement that consists of a metal powder in contact with a high-temperature volatile reducing compound is known. Regarding the chromium-based deposits, the metal powder is confined in the presence of a halide (NH4F or HF) and brought to a high temperature (950-1050° C.). Thus, deposits of chromium carbide (with the addition of hydrocarbon gas), chromium nitride (NH4F decomposing into H2 and N2, high-temperature nitrating mixture), or carbonitrides (NH4F+hydrocarbons) are obtained. This process can operate at atmospheric pressure, but the deposits are obtained only at high temperature because of the halide-type metal source that is used. The so-called “standard” CVD processes, directly using corresponding halide vapors as a chromium source, operate under dynamic vacuum and at high temperature.
These two CVD processes that are strongly activated thermally make possible the treatment of complex surfaces, but their primary drawbacks are (i) the use of thermally robust, toxic and corrosive halide precursors with limited volatility and (ii) high temperatures of deposition that, on metal substrates, will cause dimensional deformations, modification of microstructure, and changes in crystalline structure, causing the degradation, and even the loss, of specific functional properties of the tool.
To bring the deposition temperatures below 550° C., considered to be the critical holding temperature of steels and alloys, organometallic molecular precursors have been used (MOCVD process, for Metal Organic CVD). However, taking into account the low volatility and the thermal instability of these compounds that are often powders, it is necessary to operate under reduced pressure. In addition, the extended heating of the precursor in the sublimation zone, even at low temperature, can degrade the reagent before it arrives in vapor form at the deposition zone, thus involving problems of reproducibility in terms of flow rate, the initial reactive gas composition, and therefore the deposit quality.
A particular process of MOCVD deposition uses a plasma torch to thermally decompose the organometallic chromium precursors. The use of a plasma torch does not, however, make possible deposition on a heat-sensitive metal, nor does it make it possible to apply a uniform coating on large parts unless a scanning system of the plasma nozzle is provided, requiring complex technical controls. Treatment on large parts in a stream is therefore not conceivable by this technique.
Taking into account limits mentioned above of the CVD processes for deposition of protective metallurgical coatings, physical vapor-phase deposition (PVD) processes have been developed and proposed as an alternative to the CVD processes. They are generally considered to be a suitable deposition technique, without generating or using unstable, dangerous and/or corrosive chemical substances. They operate at a low temperature, which makes it possible to retain the basic specificities of the metal substrates that are to be treated. Numerous PVD processes for production of carbide-, nitride- and carbonitride-type coatings are described in the literature. Despite the investments in costly equipment, the size of the deposition frames is limited, the rate of growth is relatively low, and uniform deposition, even with target or rotating-substrate techniques, is obtained only for small parts. In all of the cases depicted, forced vacuum techniques are used, requiring sophisticated monitoring and regulation equipment. These PVD processes that bring about a low productivity are currently reserved for the treatment of lots of small high-value-added parts.
It thus appears that it has not been possible until now to carry out non-oxide ceramic depositions at low temperature and under atmospheric pressure, applicable for online treatment of large surfaces in a stream. This invention has as its object to meet this requirement. It has as its object a process that combines the principle of the chemical vapor-phase deposition and the liquid injection of a precursor of the metal compound to be deposited, called DLI-CVD for Direct Liquid Injection-Chemical Vapor Deposition.
The principle of the DLI-MOCVD technique is to introduce into a chemical vapor-phase deposition chamber a liquid precursor of the element to be deposited by periodic injection of droplets of said precursor that are entrained by a vector gas toward the deposition chamber. A DLI-MOCVD device has been developed for the deposition of thin layers of oxides on the micro-electronic plates, but it has never been used for the deposition of non-oxide ceramics at atmospheric pressure. This is due to the fact that work at atmospheric pressure, which offers a great advantage for the low-cost industrial production of large parts, imposes particular reaction conditions that have not been defined to date. Furthermore, the operating conditions are not easily transposable to the deposition of non-oxide layers. Actually, in the case of microelectronic layers, carbon contamination is to be prevented. However, whereby the latter results from the large quantity of hydrocarbon solvent in the reactor, it is more easily controlled when the decomposition is conducted in the presence of oxygen or an oxidizing gas (N2O, H2O, . . . ). Furthermore, for coatings intended for microelectronics, oxidized solvents are commonly used. A large part of the carbon is then eliminated by combustion whereas an oxide film is deposited, which is not compatible with the purpose of this invention.